Recently, there has been an increasing interest in energy storage technology day by day. As the application field of energy storage technology has been extended to mobile phones, camcorders, lap-top computers, and even electric cars, many efforts have been devoted to studying and developing electrochemical devices. In this aspect, electrochemical devices are attracting more attention, and especially, development of rechargeable secondary batteries is the focus of attention, and more recently, in the development of such batteries, new electrode and battery design to improve capacity density and specific energy are mainly studied and developed.
In the currently available secondary batteries, lithium secondary batteries developed in early 1990's have higher operating voltage and much higher energy density than traditional batteries using an aqueous electrolyte solution such as Ni-MH batteries, Ni—Cd batteries and H2SO4—Pb batteries, and by virtue of these advantages, lithium secondary batteries are gaining much attention. However, the downside of lithium ion batteries is that they have safety problem such as fires and explosion caused by the use of organic electrolyte solutions and they are complicated in manufacturing.
Novel lithium ion polymer batteries evolved from lithium ion batteries are considered one of the next-generation batteries, but lower capacity than lithium ion batteries and insufficient discharge capacity, especially, at low temperatures are issues that must be urgently solved.
Electrochemical devices are produced by many manufacturers, and each shows different safety characteristics. Assessment and management of the safety of electrochemical batteries is very grave. The most important consideration is that electrochemical devices should not cause injury to users in the event of malfunction, and for this purpose, Safety Regulations strictly prohibit fire and flame in electrochemical devices. In the safety characteristics of electrochemical devices, overheating and eventual thermal runaway in electrochemical devices or piercing of separators poses a high risk of explosion. Particularly, polyolefin-based porous substrates commonly used for separators of electrochemical devices show extremely severe thermal contraction behaviors at the temperature of 100° C. or above due to their properties of materials and manufacturing processes including stretching, causing a short circuit between positive and negative electrodes.
To solve the safety problem of electrochemical devices, a separator having a porous organic-inorganic coating layer has been proposed, in which the porous organic-inorganic coating layer is formed by coating a mixture of inorganic particles in excess and a binder polymer on at least one surface of a porous substrate having a plurality of pores.
When particles having a very large particle size are used in the separator having a porous organic-inorganic coating layer, thermal contraction control was difficult, and when particles having a very small particle size are used, a coating layer-forming composition reduces in stability or processability, making it difficult to use.